An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and to direct those beams along convergent paths to a point or small area of convergence near the tube screen. In U.S. Pat. No. 4,370,592, issued to Hughes et al. on Jan. 25, 1983, an electron gun is described wherein a main focusing lens is formed by two spaced electrodes. Each main focusing lens electrode includes a plurality of apertures therein, equal to the number of electron beams, and also a peripheral rim, with the peripheral rims of the two main focusing lens electrodes facing each other. The apertured portion of each main focusing lens electrode is located within a recess set back from the rim. The effect of this electrode structure on the main focusing lens is a gentle voltage gradient, for reducing spherical aberration.
U.S. Pat. No. 4,388,552, issued to Greninger on Jun. 14, 1983, discloses a modification in the shape of one of the peripheral rims of the above-described electron gun. In this modification, the recess in at least one of the electrodes is wider at the side beam paths than at the center beam path, measured perpendicular to the plane containing the inline electron beams. This modification redistributes the electrostatic field lines of the main focusing lens, so that the focus voltages for the three beams are unitized.
U.S. Pat. No. 4,626,738, issued to Gerlach on Dec. 2, 1986, discloses a main focusing lens formed by two electrodes, each of which includes an outer oval-shaped part with a peripheral rim. The rims of each electrode face each other. Telescoped within each oval-shaped part is an apertured plate that has a corresponding oval-shaped periphery. In this type of main lens construction, it has been found that the distance between the apertured plates and peripheral rims may vary unless extreme care is taken during fabrication of the electron gun. Furthermore, it is also possible to insert the apertured plates at an angle that is slightly out of alignment with the peripheral rims. The present invention provides an improved construction for the main focusing lens electrodes in the type of electron gun that utilizes such apertured plates.